Metallic liquid normally enters an extremely unstable state when cooled below a melting point, and is immediately crystallized to become crystallized metal. In this event, time for which a supercooled liquid can exist in an uncrystallized state where atoms are randomly arranged, i.e., a so-called “amorphous state,” is estimated to be 10−5 seconds or less at a nose temperature of a continuous cooling transformation (CCT) curve. Specifically, this means that it is impossible to obtain amorphous alloys unless a cooling rate of 106 K/s or more is achieved.
However, there has recently been invented metallic glass which undergoes clear glass transition and is not crystallized even at a cooling rate of 100 K/s or less since a supercooled liquid state is extremely stabilized in a specific alloy group including a zirconium base (see, for example, The June 2002 edition of Kinou Zairyou (Functional Materials), Vol. 22, No. 6, p.p. 5-9; Non-patent Document 1).
Since the metallic glass has a wide supercooled liquid temperature range, superplastic forming utilizing a viscous flow is possible while under conditions that do not reach a temperature and time at which the glass is transformed into crystals again. Thus, the metallic glass is expected to be put into practical use as a structural material.
Among the metallic glass, as in the case of commercial titanium used as a structural material, zirconium-based metallic glass containing zirconium as a basic component, having a high affinity for oxygen, has been expected to have its surface colored in several colors depending on its thickness by forming an oxide film on the surface.
For example, Japanese Patent Publication No. 2003-166044 (Patent Document 1) discloses a method of toning a surface of zirconium-based amorphous alloy in brown with a thickness of 0.1 μm or less, in black with a thickness of 0.1 to 8 μm and in gray with a thickness of 8 μm or more by subjecting the zirconium-based amorphous alloy to heat treatment in the atmosphere. The method proposed here is basically a method by which surface oxidation by heating at 350° C. to 450° C. in the atmosphere is expected.
However, in the method described in Patent Document 1, it is impossible to manage an oxide film in order that the entire zirconium-based metallic glass component can be evenly colored. Moreover, the type of color obtained is limited to brown, black or gray. Thus, the method has a problem that a decorative surface desired for the zirconium-based metallic glass component is extremely limited.
Furthermore, in the method described in Patent Document 1, heating and oxidation in the atmosphere tend to accelerate crystallization of a normally amorphous surface layer more than desired. Thus, the method also has a problem that the zirconium-based metallic glass component becomes fragile unless an amorphous structure of the surface layer of the entire zirconium-based metallic glass component is maintained and controlled by strictly managing both the temperature and time.
Consequently, in order to solve the problems described above, the inventors of the present invention have carried out numerous studies for the purpose of coloring the surface of the zirconium-based metallic glass component. As a result, the inventors have found out that it is possible to perform coloring in many colors without worrying about crystallization depending on the temperature by carrying out an anodizing process to form an interference film. Moreover, the inventors have also found out that it is possible to produce many colors without causing crystallization by heating while controlling an inert gas atmosphere. Furthermore, the present invention has been accomplished by optimizing conditions for formation of the film.